1. Field of the Invention
The present invention relates to a sand classification tank for producing classified sand, i.e., sand containing specific and predetermined fractions of differently-sized sand grains per unit of volume.
2. Related Art
Conventional equipment for classifying sand for use in pour concrete comprises a tank partially filled with water, and a mixture of sand and water is introduced into the tank below the surface of the water through a feed inlet. The grains of sand follow a natural current of flow through the tank. The sand is deposited as a sediment at the bottom of the tank, the sedimentation being determined by gravity according to the weight of the various grains. Thus, the sand is deposited along the length of the tank, the coarsest sand fraction being deposited closest to the feed inlet and finer particles farther downstream. The finest fraction, which is called "fines", is deposited farthest from the feed inlet--assuming that these particles are able to settle at all, being very buoyant and thus not readily sinking to the bottom of the tank. In the prior art sand classification tanks, the various sand fractions are metered out through hydraulically operated valves at the bottom of the tank, in the form of plate valves. Excess water usually has to be drained off in order to facilitate further transport of the sand fractions, so the mass leaving the tank normally passes over dewatering wheels and the like. Before the unclassified sand enters the tank, it passes through a sieve which holds back larger particles. The prior art systems cannot handle particles of larger diameter than about 5 mm. Sand grains that are larger than this, for example 10 mm in diameter, may cause serious damage to the prior art equipment, because if these particles manage to penetrate between the bottom of the tank and the valve at the bottom, the valve will not operate properly and a permanent water leakage results.
Recent research has shown that a substantial savings of cement and a better concrete are obtained if the classified sand contains a controlled amount of fines, i.e., grains of sand between about 0.002 and 0.006 mm in size. It is also desirable to increase the diameter of the coarsest fraction up to a grain size of about 12 mm diameter.
The prior art systems cannot handle sand grains larger than about 5 mm owing to the risk of leakage through the bottom valves. Another drawback of prior art classification tanks is that a considerable proportion of the fines is washed away during the process, being drained off together with the waste water. This occurs when the bottom valves are opened for conducting the mass down to the dewatering wheel--which, it might be added, is an expensive structural component. When the mixture of water and sand reaches the dewatering wheel, the resulting turbulence causes some of the fines to be washed out in the overflow water. Known equipment of this type also requires vast amounts of water to function, on the order of 3,000-4,000 liters/minute.